This invention relates to an improved process for the separation of an ethylenically unsaturated hydrocarbon from a hydrocarbon mixture containing same.
Various hydrocarbon mixtures are obtained from the thermal cracking of petroleum products such as naphtha, gas oil, light oil, crude oil, etc. A typical hydrocarbon mixture from cracking operations is pyrolysis gasoline which contains generally aromatic and cycloparaffinic compounds having 5-10 carbon atoms. A typical hydrocarbon mixture obtained following the removal of the hydrocarbons containing 5 carbon items is given in Table A:
TABLE ______________________________________ Compound % by Weight ______________________________________ Non-aromatics 16-18 Benzene 33-37 Toluene 16-20 Ethylbenzene 1-2 p/m Xylenes 5-7 o-Xylene 2-3 Styrene 6-8 Dimethylcyclopentadiene &lt;1 C.sub.9 Aromatics .about.1 .alpha.-Methylstyrene &lt;1 Vinyltoluene 2.5-3 Indene 2.5-3 Methyl indene &lt;1 Naphthalene &lt;1 Phenylacetylene 0.1 or less ______________________________________
It should be understood that the above hydrocarbon mixture is used to illustrate a typical hydrocarbon mixture and is not presented to define such mixture since such hydrocarbon mixtures may vary greatly.
One of the commercially more valuable components found in the above mixture is styrene. An economical means for separating out this preferred component has been long sought after. The removal of styrene from these mixtures by fractional distillation is rendered unfeasible owing to the presence of other components in the hydrocarbon mixture, especially o-xylene, which have boiling points very close to that of styrene. Table B presents the boiling points of styrene and other hydrocarbons.
TABLE B ______________________________________ Compound Boiling Point .degree.C. ______________________________________ Methylstyrene 163.4 n-Propylbenzene 159.2 Cumene 152.4 Cyclooctane 148.5 STYRENE 145.2 O--Xylene 144.4 3-Methyloctane 143-144 Cyclooctene 138-143 Phenylacetylene 142.4 4-Methyloctane 141-142 m-Xylene 139.1 p-Xylene 138.4 Ethylbenzene 136.2 Toluene 110.6 ______________________________________
One of the more widely used processes for utilizing this styrene involves hydrogenating the styrene to ethylbenzene and thereafter separating it from the xylenes by precision fractional distillation. Following this distillation, the ethylbenzene is then dehydrogenated to styrene and again purified by another distillation. This method is very complicated and expensive. The disadvantages of the aforementioned method have promoted research concerning the direct separation of styrene from the hydrocarbon mixture without first converting the styrene to ethylbenzene.
British Pat. No. 1,038,606 proposes a process utilizing an aqueous solution of a silver salt such as AgNO.sub.3 to extract styrene from a hydrocarbon mixture following treatment of the mixture with fuller's earth to prevent slime formation. This method has the disadvantage of being expensive due to the use of silver salts.
Also, U.S. Pat. No. 3,328,267 proposes a process consisting of extractive distillation for the separation of styrene from o-xylene using a di-lower-alkyl formamide such as dimethyl formamide as the extractive distillation solvent. This process also makes use of a polymerization inhibitor such as quinone or hydroquinone or preferably p-tert-butylpyrocatechol. However, the styrene produced by this process has the undesirable characteristic of being light yellow in color. Also, the polymerization inhibitor requires low temperatures to be effective.
U.S. Pat. No. 3,580,839 (Fuerst) entitled, "Recovery of Aromatic Hydrocarbons from Mixtures of Hydrocarbons by Selective Extraction with a Substituted Piperazine Solvent", describes the use of piperazine derivatives having the general formula: ##STR1## where X denotes formyl or acetyl and R denotes lower alkyl, as extractants for recovering mono- or di-nuclear aromatic hydrocarbons which may have lower (1-4 C) aliphatic hydrocarbon radical substituents from hydrocarbon mixtures. Thus, this process proposes the separation of a hydrocarbon mixture into two fractions; one fraction containing predominantly aromatic hydrocarbons such as benzene, toluene, xylene, ethylbenzene and styrene; and another fraction having a residue predominate in paraffins, olefins or cycloparaffins. The piperazine derivatives may be used in a pure form or mixed with other extractants such as tetramethylsulfone, diethylene glcyol or triethyl glycol. N-formyl-N'-methyl piperazine is the preferred extractant. This process is directed toward a gross separation of products.
In U.S. Pat. No 3,684,665, Abe et al, proposes a method for separating styrene from hydrocarbon mixtures containing xylenes which comprises extractively distilling a hydrocarbon mixture with a suitable solvent causing the styrene to be concentrated in the solvent from which it can then be removed. Suitable solvents include dialkyl acetamides such as dimethylacetamide, as well as dialkylsulfoxides, alkylene carbonates, lactones, lactams, phenol, alkylphenols, salicyclic acid, alkyl esters, aniline, alkyl anilines, phthalic acid alkyl esters, tetraalkyl ureas, N,N-dialkyl carbamic esters and glycol monoalkyl ethers such as diethylene glycol monoalkyl ether and N-methyl-pyrrolidone. Use of a polymerization inhibitor such as hydroquinone, tert-butylcatechol, phenothiozine, sulfur or mixtures thereof, is urged to prevent polymerization of the styrene. This process, however, suffers from the disadvantages of polymerization losses especially at temperatures above 100.degree. C. and the production of styrene that is undesirably yellow in color.
In U.S. Pat. No. 3,763,015, Morimoto et al, present another extractive distillation process utilizing a polar organic solvent in the presence of a nitrile polymerization inhibitor. Following extractive distillation, the solvent containing styrene is treated with nitric acid and then again distilled to remove impurities and separate the styrene from the solvent. Suitable solvents for this process are given as diethylacetamide, .beta.-methylpropionitrile, butyl lactone, N-methylpyrrolidone, dimethylformamide and dimethylsulfoxide. The preferred polymerization inhibitors are sodium nitrite or potassium nitrite used in combination with a compound having at least one nitro, nitroso, quinoide, phenolic or hydroxy group in the molecule. The preferred additives are p-tert-butyl-catechol, hydroquinone, p-benzoquinone, p-dinitrosobenzene, .alpha.-nitro-.beta.-naphthol, o-nitrosonaphthol and .alpha.-naphthoquinone. This process produces a yellow styrene product requiring further treatment with nitric acid to remove the colored impurities. Also, yield losses through polymerization are a problem.
In view of the aforementioned deficiencies of the prior processes, it is highly desirable to provide a process for effectively separating a monovinylidene aromatic or other ethylenically unsaturated hydrocarbon from hydrocarbon mixtures containing same while coincidently inhibiting polymerization to produce a substantially pure, color-free product.